1)as， when 引導(dǎo)短暫性動(dòng)作的動(dòng)詞。例如：

Just as / Just when / When I stopped my car， a man came up to me. 我一剎車，有一個(gè)人向我走來(lái)。

2)當(dāng)從句的動(dòng)作發(fā)生于主句動(dòng)作之前，只能用when 引導(dǎo)這個(gè)從句，不可用as 或 while.例如：

When you have finished your work， you may have a rest. 趕完活后，你可以休息一下。

3)從句表示"隨時(shí)間推移"連詞能用as，不用when 或while.例如：

As the day went on， the weather got worse.日子一天天過(guò)去，天氣越變?cè)綁摹?/p>